The present invention concerns a magnetic recording medium for realizing a high density magnetic recording, and a magnetic memory device of a large capacity using the same.
Demand for larger capacity has increased more and more for magnetic disk apparatus. For coping with the demand, development has been required for a magnetic head at high sensitivity and a recording medium of high S/N. For improving S/N of the medium, it is necessary to improve the read output when recorded at high density. Generally, a recording medium includes a first underlayer referred to as a seed layer (formed on a substrate), a second underlayer of a body-centered cubic structure including chromium as a main ingredient, a magnetic layer, and a protective film including carbon as a main ingredient. For the magnetic layer, an alloy having a hexagonal close packed structure mainly including cobalt as a main ingredient is used. For improving the read output, it is effective to crystallographically orient the (11·0) face or (10·0) face substantially in parallel with the plane of the substrate in the magnetic layer and direct the c-axis of the hexagonal close packed structure as the easy axis of magnetization to the in-plane direction. It has been known that the crystallographic orientation of the magnetic layer can be controlled by a seed layer. Further, it has been known that the read output can be improved by applying mechanical texturing to the substrate thereby introducing magnetic anisotropy in the circumferential direction (see Patent document 1, Japanese Patent Laid-Open No. 5-205239).
In addition to the improvement for the read output, decrease of medium noises is also an important subject in view of the improvement of the medium S/N. For decreasing the medium noises, it is effective to adopt a multi-layered structure for the magnetic layer, refine the grain size and decrease Brt as a product of a residual magnetic flux density (Br) and the film thickness (t) of the magnetic layer. That is, it has been provided a magnetic recording medium of forming an underlayer on a substrate, and disposing thereon stacked magnetic films constituted with at least two magnetic layers of different compositions in contact with each other by way of a non-magnetic layer such as made of ruthenium in a multi-layered constitution (see Patent document 2, Japanese Patent Laid-Open No. 7-134820). By the use of the magnetic recording medium described above, a magnetic recording medium of a multi-layered structure intended for decreasing the noises while maintaining magnetic characteristics having a thermal stability is attained.
Since excess refinement of the grain size and decrease of Brt result in the degradation of the thermal stability, decrease of noises is limited and higher coercivity of the medium has been considered together. Although the half amplitude pulse width PW50 of the isolated read pulse output can be improved shorter by increasing the coercivity, this tends to result in determination of the overwrite characteristic and compatibility between the high coercivity and the overwrite characteristic is limited.
In recent years, an anti-ferromagnetic coupling medium has been proposed as the technique of compatibilizing the thermal stability and the decrease of noises (see Patent document 3, Japanese Patent No. 3421632). This adopts a dual layered structure of anti-ferromagnetically coupling magnetic layers by way of a Ru intermediate layer and can set Brt lower while leaving the magnetic film thickness larger compared with a medium including a single magnetic layer. Accordingly, the medium noises can be decreased while maintaining the thermal stability. However, the combination of the techniques described above is still insufficient for attaining an areal recording density of 95 Mbits or more per 1 mm2 and it is necessary to further improve the read output and decrease the medium noises.
To address these concerns, Patent document 4 (Japanese Patent Laid-Open No. 2003-85729 (FIG. 3)) proposes a magnetic recording medium having at least a first magnetic layer including a ferromagnetic material for controlling the anti-ferromagnetic exchange interaction, a second magnetic layer including a ferromagnetic material and a spacer layer formed between the first magnetic layer and the second magnetic layer for inducing the ferromagnetic exchange interaction on a substrate in which the second magnetic layer includes plural layers.
In Reference Example 1 described in page 7 of Patent document 4, the thickness of the lower magnetic layer 71 using a Co-20 at. % Cr-9 at. % Pt-3 at. % Ta alloy is set to 80 Å, and the thickness of the upper magnetic layer 72 using a Co-20 at. % Cr-12 at. % Pt-7 at. % B alloy is set to 70 Å. It is shown that since the thickness of the lower magnetic layer 71 is made larger than that of the thickness for the upper magnetic layer 72 while the coercivity Hc is lowered, the coercive squareness (S*), and the overwrite characteristic (O/W) are greatly improved and, further, pulse width (PW) is also improved compared with an AFC structure not provided with the lower magnetic layer 71.
Further, by interposing the lower magnetic layer of thin film thickness between the spacer layer and the upper magnetic layer, disturbance in the crystallographic orientation can be prevented in a case of forming the upper magnetic layer directly on the spacer layer. While this improves the coercive squareness (S*) and the pulse width (PW), the medium noises (S/N ratio) and thermal fluctuation characteristic are sometimes deteriorated. Since the lower magnetic layer having deviation for the lattice constant from that of the spacer layer includes disturbance of the crystallographic orientation, increase of the thickness for the lower magnetic layer causes increase of noises. Accordingly, it is shown that the lower magnetic layer is desirably as thin as possible in a state of keeping the effect of improving the orientation of the upper magnetic layer.